The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Electric motors are used in many applications. For example, single-phase and two-phase electric motors are used to drive cooling fans in electronic equipment. A fan may provide optimum airflow when the fan rotates in one direction as opposed to the other. This may primarily be due to the blade/wing design of the fan. Therefore, it may be efficient and desirable to rotate the fan in the direction that has maximum airflow for a given speed.
The speed of a motor can be determined in many ways. For example, Hall-effect sensors can be used to detect the speed of the motor. The Hall-effect sensors, however, increase the cost of cooling fans. Alternatively, the speed of the motor can be determined by sensing back-EMF of the motor. The back-EMF alone, however, can be insufficient to determine the direction in which the motor rotates.